Wireless communications devices, a wireless telephone or laptop computer with a wireless transponder for example, are known to use simple cylindrical coil antennas as either the primary or secondary communication antennas. The resonance frequency of the antenna is responsive to its electrical length, which forms a portion of the operating frequency wavelength. The electrical length of a wireless device helical antenna is often a ratio such as 3λ/4, 5λ/4, or λ/4, where λ is the wavelength of the operating frequency, and the effective wavelength is responsive to the dielectric constant of the proximate dielectric.
Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS) at around 850 megahertz (MHz), and the PCS (Personal Communication System) band at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz, the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are global positioning satellite (GPS) signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz.
Wireless devices that are equipped with transponders to operate in multiple frequency bands must have antennas tuned to operate in the corresponding frequency bands. Equipping such a wireless device with discrete antenna for each of these frequency bands is not practical as the size of these devices continues to shrink, even as more functionality is added. Nor is it practical to expect users to disassemble devices to swap antennas. Even if multiple antennas could be designed to be co-located, so as to reduce the space requirement, the multiple antenna feed points, or transmission line interfaces still occupy valuable space. Further, each of these discrete antennas may require a separate matching circuit.
For example, an antenna can be connected to a laptop computer PCMCIA modem card external interface for the purpose of communicating with a cellular telephone system at 800 MHz, or a PCS system at 1900 MHz. A conventional single-coil helical antenna is a good candidate for this application, as it is small compared to a conventional whip antenna. The small size makes the helical antenna easy to carry when not attached to the laptop, and unobtrusive when deployed. The single-coil helical antenna has a resonant frequency and bandwidth that can be controlled by the diameter of coil, the spacing between turns, and the axial length. However, such a single-coil helical antenna will only operate at one of the frequencies of interest, requiring the user to carry multiple antennas, and also requiring the user to make a determination of which antenna to deploy.
Multiband antennas have been built to address the conflicting goals of a small size and the ability to operate in multiple frequency bands. Dipole antennas are inherently larger due to a two-radiator design. Other antenna designs, with an inherently smaller form factor, are sensitive to the relative position of other radiators and the grounds. Sub-optimal performance is often due to the positional change of the ground planes relative to the antenna. An antenna that depends heavily on the ground plane, such as a patch antenna, planar inverted-F antenna (PIFA), or folded monopole, may perform poorly when a grounded metal is near the antenna in some configurations. Likewise, the performance of one, ground-dependent, radiator is susceptible to proximately located radiators.
Some multiband antenna designs are made smaller by connecting the radiating elements in series. However, any change to one of the radiators affects the performance of others in the series. This interdependence between radiators makes the antenna difficult to design. In use, all the radiators can be affected, even if only one radiator becomes detuned due a proximate object or changing groundplane.
Poor antenna performance can be characterized by the amount of current unintentionally generated through a transceiving device, typically as surface currents, as opposed to amount of energy radiated into the intended transmission medium (i.e., air). From the point of view of a transmitter, poor antenna performance can be measured as less radiated power, or less power in an intended direction. From the receiver perspective, poor antenna performance is associated with degraded sensitivity due to noisy grounds. From either point of view, poor performance can be associated with radio frequency (RF) ground currents.